In 2D animation, how to draw coloured shapes?
I found the answer shortly after posting the question for the second time. For everybody who is puzzled about this as well, follow this description

By default, 4 materials are visible in the material box at the top of the screen (the one that drops down when you click on "Materials"): Black outline, red outline, grey filled, and black dotted outline. To make it possible to fill a shape with another color you need to add a new material. 



To add a new material, open the "Material Properties" window at the right side, by clicking on the little red ball icon:



After that, press on the '+' sign in the corner, and a grey bar appears in the list. Then press the 'New' button:



In the properties that then appear, change the default 'Material' name to anything you like, and enable the 'Fill' property. Then click on the 'Color', which is black by default, and a color selection dialog appears, with a black color wheel. The easiest then is to use the color selection 'thermometer' to sample the color you want. Or sample any other color at all, except black, and then select from the color wheel:



I have not found an easy way to do this.